1. Field of the Invention
The present invention generally relates to a composition and process for extracting metals from liquids. More particularly, this invention relates to a composition and process for extracting cesium and other metals from alkaline waste solutions, including solutions that are highly concentrated in salts. The present Application claims the benefit, under Title 35 U.S.C. .sctn.119(e), of United States Provisional Application Serial No. 60/057,974 entitled "Solvent and Process for Extracting Cesium from Alkaline Waste Solutions," hereby incorporated by reference.
Many nuclear energy complexes and treatment sites have environmental problems where cesium removal is needed. Alkaline wastes containing radioactive Cesium-137, such as those stored at the Department of Energy's Hanford. Wash., Oak Ridge, Tenn., and Savannah River, S.C. sites are examples.
No one satisfactory technology for the removal of cesium has been found due to disadvantages inherent to current methods. Solid phase sorbents (inorganic ion exchangers) do not remove and sorb cesium to a concentration sufficient to permit the cesium-loaded sorbents' ready incorporation into a waste form such as borosilicate glass in an economical or practical manner. Inorganic ion-exchange materials such as ammonium molybdophosphate, and crystalline silicotitanate can sorb cesium from dilute aqueous stripping solutions, but not from highly concentrated and alkaline raw wastes. What is more, ammonium molybdophosphate cannot be used to treat raw alkaline waste, as the material starts to dissolve at a pH above 6.
Solvent extraction processes on the other hand, contribute greater flexibility to the overall problem of treating the waste and encapsulating the cesium in a waste form. Nevertheless, there are currently no practical solvent extraction processes for the removal of cesium directly from the tanks with the waste in high salt alkaline form that the Applicants are aware of, only acid-side extraction has been addressed, and the use of acid-side solvent-extraction technology to treat the alkaline tanks would require acidification of the waste--a costly option.
Accordingly, the present invention alleviates the necessity of adding acid or other substances to the waste since cesium extraction may be effected directly from the waste matrix. As such, the present invention could play a key role in a grand treatment scheme for alkaline nuclear wastes, especially wastes with a high concentration of competing alkali metal cations.
A further problem to be solved is the need for a method which regenerates the extractant by utilizing a safe and cost-effective stripping procedure and which avoids further generation of waste. Such method should also release cesium from the extractant solvent without employing highly concentrated mineral acids, solvent evaporation or distillation, or contacting of the solvent with cation exchangers. Thus, the present invention comprises a solvent extraction and stripping process cycle for the removal of cesium from alkaline tank waste. After the solvent is stripped of cesium, the solvent can be recycled in a continuous extraction and stripping process cycle.
Previously reported extractants have generally possessed insufficient selectivity or extraction power to remove cesium from a matrix concentrated in competing alkali metal cations. In addition, earlier extraction solvents involved difficulties with stability, stripping, or phase disengagement. Thus, no other candidate solvent system has emerged as a serious contender for the targeted application.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
In "Cesium Extractive Metallurgy: Ore to Metal," Journal of Metals, 1966, pp. 1198-1202, K. C. Dean et al stress the need for methods to separate cesium from cesium ore and describe prior art used, including leaching and solvent extraction. Even so, the extractants tested there were of limited effectiveness. Among them was the compound 4-sec-butyl-2-(.alpha.-methylbenzyl)phenol (known by the acronym "BAMBP"). Although it was extensively investigated in the 1960's for potential use in extracting cesium from alkaline waste solutions, development of this material for a solvent extraction process for cesium was eventually abandoned due to a number of undesirable attributes, most notably a lack of solubility of the cesium-BAMBP complex at high cesium loadings. BAMBP also exhibited precipitation problems during the running of continuous liquid-liquid contacting, which also made it unsuitable for such processes.
Blasius et al teach the use of macrocylic polyethers for the extraction of Cesium in U.S. Pat. No. 4,647,440 ('440). This method requires an inorganic complex salt/acid to act as an extractant in tandem with the ether. The liquid phase adduct formulation described therein is not soluble in a non-polar organic diluent. For liquid-liquid extraction it was in fact only successful with a polar organic diluent, since otherwise neither the macrocyclic ether nor the inorganic complex acids/salts were soluble. The lack of phase coalescence for the liquid-liquid extraction method was problematic. It may be noted that the preferred diluent in most hydrometallurgical processes, including nuclear applications, consists of aliphatic hydrocarbons. Thus, the use of polar diluents is generally not acceptable.
In "Selective Extraction of Cesium from Acidic Nitrate Solutions with Didodecylnaphthalenesulfonic Acid Synergized with Bis(tert-butylbenzo)-21-crown-7." Anal. Chem., 64 (1992), pp. 3013-3017. W. Jack McDowell et al, teach the use of other types of crown ethers for extracting cesium from acidic solutions, but with limited success. These solvent extractions involved a different class of crown ethers than those in the present invention. Disadvantages included the use of a non-aliphatic diluent, inadequate selectivity, and large consumption of acid for stripping.
R. Ludwig details several applications of calixarene-type macrocycles in the general report entitled, "Review on Calixarene-Type Macrocycles and Metal Extraction Data." Report JAERI-Review 95-022, Japan Atomic Energy Research Institute (1995). While it provides a good comprehensive survey of the literature, it shows that no calix-crowns of the type reported in the present invention have been used for the extraction of cesium. It also shows no prior applications which included investigations of alkaline nitrate solutions.
Several other papers specifically relate to the extraction of cesium using calixarene-crown ether compounds. These include Z. Asfari et al, "Doubly Crowned Calix[4]arenes in the 1,3-Alternate Conformation as Cesium Selective Carriers in Supported Liquid Membranes," Analytical Chemistry, 67 (1995), pp. 3133-3139; A. Casnati et al, "Synthesis, Complexation, and Membrane Transport Studies of 1,3-Alternate Calix[4]arene-crown 6 Conformers: A New Class of Cesium Selective lonophores," J. Am. Chem. Soc., 117 (1995), pp. 2767-2777; C. Hill, "Application Des Calixarenes Fonctionnalises au Traitment des Effluents Radioactifs par Membranes Liquides Supportees," Ph.D. Thesis, Universite Louis Pasteur de Strasbourg, 1994, Chapter 2; C. Hill, et al, "Nuclear Waste Treatment by Means of Supported Liquid Membranes Containing Calixcrown Compounds," J. Incl. Phen. Mol. Recog. Chem., 19 (1994), pp. 399-408; and Rocco Ungaro et al, "1,3-Dialkoxycalix[4]arenecrowns-6 in 1,3-Alternate Conformation: Cesium Selective Ligand that Exploit Cation-Arene Interactions," Angew. Chem. Int. Ed. Engl., 33 (1994), pp. 1506-1509. Each of the above papers and the dissertation concern the use of calix-crown compounds for cesium separation from aqueous solution, including nuclear waste. Yet all experiments described in these works dealt with acidic or neutral solutions containing no, or very low, concentrations of competing metal cations. No calix-crowns of the type disclosed in the present invention were reported. It should also be noted that the investigations of separation ability in these references aimed primarily at the use of supported liquid membranes.
Some solvent extraction data are presented in these works especially in the dissertation of Hill. Some of the calix-crowns Hill discusses in his thesis are sparingly soluble in the aromatic diluents 1,4-di-isopropylbenzene and n-hexylbenzene. However, the cesium extraction was very low (the cesium distribution ratio was on the order of 1.0.times.10.sup.-2 from an acidic feed solution composed of 1 M HNO.sub.3 and 5.times.10.sup.-4 M CsNO.sub.3), and in many cases the calix-crown was observed to precipitate from the solvent (Hill, 1994). Diluents in which these materials are soluble, such as nitrobenzene, chloroform, ortho-nitrophenylpentyl ether (NPPE), ortho-nitrophenylhexyl ether (NPHE), and ortho-nitrophenyloctyl ether (NPOE), are either too toxic (nitrobenzene, chloroform), or too expensive (NPPE, NPHE, NPOE) to be useful or practical for an actual solvent-extraction process to extract cesium from nuclear waste on a large scale. As mentioned above, aliphatic diluents are preferred for industrial use. There has also been a tendency for diluents incorporating these extractants to form undesirable precipitates or third phases when contacted with solutions containing high concentrations of alkali metal cations.
In French Patents 92 14245 and 93 04566, Dozol et al teach a method and compound relating to the extraction of cesium using calixarene-crown ether compounds. They describe the synthesis and use of specific Calix[4]arene mono-crown and bis-crown ethers dissolved in alkyl benzene or nitrophenyl alkyl ethers (hereinafter NPOE and NPHE) at 0.001 to 0.50 M for the extraction of cesium from nitric acid solutions from 10.sup.-3 to 7 M. The calix-crowns that are claimed do not, however, include the catagory of calix-crown ether described in the present invention, and no mention is made of their use in process-suitable aliphatic hydrocarbon diluents. This is presumably due to their lack of solubility and weak extractability in these diluents. Examples describing extraction of cesium from a matrix of 0.97 to 1.0 M nitric acid and 5.times.10.sup.-4 M cesium using the calix-crowns dissolved in NPHE at 0.01 M were given, and recovering the cesium in the organic phase by contacting (stripping) the organic phase with water was mentioned. But no mention of extraction from alkaline solutions or solutions containing high concentrations of competing ions is presented. Though the claims discuss recovering cesium from "une solution aqueuse" (an aqueous solution with no mention of its pH), they are rather ill-defined in this regard and are not supported by use with alkaline or high salt solutions.
In "Applicability of a Calixarene Crown compound for the Removal of Cesium from Alkaline Tank Waste," Radiochimica Acta, 1997, 76, 103-108, Haverlock et al describe the use of calix[4]arene-bis-(2,3-naphtho-crown-6) at 0.01 M in orth-xylene, 1,2-dichlorobenzene, 1.2-dichloroethane, and nitrobenzene, for the separation of cesium from alkaline Hanford Tank waste simulants by solvent extraction. This calix-crown, however, is not soluble in liquid--liquid extraction process-suitable aliphatic diluents. It was in fact the lack of solubility of this and other known calix-crown materials that drove the research to develop the aliphatic-soluble calix-crown molecules described in the present invention.
Hence, new technology is required to meet the existing and future operating characteristics of methods and compositions designed to selectively extract cesium and other metals from aqueous alkaline waste solutions. There is also great need for an improved method and composition for the similar removal of cesium and other metal contaminants from neutral and acidic aqueous waste solutions.